The present invention generally relates to methods and devices for measuring the value of a capacitive element, and especially to detecting small variations in the value of a capacitor about a reference value. More particularly, it relates to a new and improved single step method for directly sampling a capacitor to measure its value, to new and improved detector circuits for performing the measurement, and to new and improved methods in which successive measurements are taken over time and in which trends in the variations of the sample values are identified, analyzed and used to formulate command signals or decisions for a wide variety of end uses. In accordance with one preferred embodiment, the present invention provides a new and improved environmentally sealed, no-touch-required, solid state keypad device for providing a new and improved human-to-machine control command input interface adapted for a large number of specific industrial and commercial applications.
There are many different and well known and documented methods of capacitance measurement. Illustrative examples include bridging methods, which are based on comparing the value of the capacitor in AC permanent regime with the value of a reference impedance. There are many variations of the bridge circuit used to measure capacitors, the best known ones are Sauty, Wien, Nernst, and Schering. Resonance methods are also employed in measuring small capacitors by working in AC permanent regime and applying a tuned circuits theory to derive the unknown value of the capacitor.
Indirect prior art methods have been based on including the capacitor under test in a reaction loop of an oscillator and measuring the resulting frequency shift. Moreover, prior methods have also included introducing the capacitor under test in an active filter configuration and measuring the transfer characteristic of the circuit. Other direct methods have included DC methods based on charging/discharging a capacitor under test with a precisely controlled constant current source, thereafter measuring either the voltage across the capacitor after a specified amount of time, or measuring the necessary time required to reach a specified voltage.
Direct methods have included AC methods which measure directly the reactance of the capacitor under test using AC generators and AC voltmeters or amperemeters and apply a generalized Ohm's law.
Small variations in the value of capacitor, however, are difficult to measure using traditional techniques. There are many applications where knowing the exact value of a capacitor is less important than quantifying its relative variance over time. General purpose measurements in prior devices, such as capacimeters having a resolution of 1% or better, are slow and relatively expensive.
Aside from the need in certain applications for measuring the value of an unknown capacitance solely for the determination of its value in Farads, it is often advantageous to utilize capacitance as a parameter in the determination or variation of another variable. For example, a change in capacitance may readily be utilized as a method for entering data into a system, for example.
In this regard, the idea of a capacitive keyboard is not new. There have been many attempts in the prior art to introduce the benefits of a solid state keyboard with no moving parts into the marketplace. These benefits include aesthetics, i.e., the keys can work from behind a front panel of the device so that the keyboard blends into the surface, ruggedness, resistance to shock, and long service life. A rather basic and increasingly important example of this type of data entry is found in the use of capacitive touch sensors in systems where it is desirable to provide substantial isolation between the equipment operator and the system control circuits. An example of this type of input control pad may be found in machinery control panel applications where it is desirable to provide a significant degree of environmental safety, not only for the electronic circuitry incorporated into the control panel, but also with respect to a human operator inputting commands through the keypad. Most prior art capacitive keyboards have suffered from extreme environmental sensitivity wherein environmental changes cause false keystroke detection.
Prior efforts to provide capacitive keyboards which are self-correcting for environmental changes are described in Eichelberger et al.'s related U.S. Pat. Nos. 4,039,940; 4,145,748; and 4,290,052. The Eichelberger et al. capacitive keypad systems incorporate a circuit for digitizing the measured analog signal value of each keypad in a sensor array. Repeated samplings of the condition of the keypad reveal detected changes in the value of a given key. If the changes are above a predetermined threshold amount, a touch indication is given. The array is subjected to periodic calibration cycling which raises or lowers the predetermined threshold limit for detecting an intentional touch based on a corrected baseline value for the capacitor cell under a no-touch condition. The calibration cycling can increment or decrement the baseline value of the capacitor one step in either direction. Although the capacitive touch entry systems described by Eichelberger et al. tend to be self-optimizing and to interactively correct for changes in environmental factors, the single step corrections contemplated in these patents are not sufficiently adapted or fast enough to correct for commonly encountered environmental changes.
Accordingly, to overcome the deficiencies in the prior art devices, it is an object of the present invention to provide a new and improved capacitive measurement method and apparatus which directly measures the value of an unknown capacitance in a single step.
It is another object of the present invention to provide a new and improved proximity sensor for sensing the approach, nearness, and retreat of an object with respect thereto.
It is a further object of the invention to provide a new and improved human to machine input interface in the form of a new and improved keypad which is substantially unaffected by changes in environmental conditions surrounding the keypad location, which does not require physical contact between the operator and the active keypad elements, which continually and automatically recalibrates its field sensitivity to compensate for environmental changes and component aging and which is capable of discerning between an intentional keystroke and other sources of a variation in key value, for example, an accidental double key press, impact of an object other than a finger, such as a ball or a bird.
It is still another object of the present invention to provide new and improved methods for reducing noise from an analog electrical signal and for providing dynamic adaptive signal tracking to identify and adjust for environmental factors.
It is another object of the present invention to provide a new and improved method for detecting variations in an electrical signal above a predetermined amount in the presence of noise.